This invention relates in general to measuring devices and methods and in particular to a new and useful device and method for measuring screw threads.
Both individual devices and automatic measuring arrangements are known for measuring screw threads. The individual devices are primarily employed for measuring the diameter, depth, flank steepness, form and taper of internal as well as external threads. Of the individual devices that are known, primarily dial and slide gauges are used. Even though such devices are handy and precise in use, they are not very suitable for series measurements, since they require time and are expensive.
Aside from these individual devices, fully automatic systems are known for measuring the minor diameter, taper, pitch and depth of exterior threads. Particular requirements are imposed on automatic measuring systems if external threads of large tubes for oil pipelines are to be measured in rapid succession on a large scale with precision.
Because of their dimensions, such tubes are difficult to handle and consequently, not easy to measure. Several manual measuring systems exist which require an operator or the like to be heavily involved in all of the steps of measurement.
U.S. Pat. No. 1,281,805 issued to Metzger teaches a workpiece with an article having an external thread placed in a corresponding holder and moved manually, by a gear drive with respect to measuring probes. The measuring probes 23 and 24 are installed on a stationary head as a flat plate 10 for the probe 23 and as a plate 29 movable on the head for the probe 24. The probe 23 penetrates into the thread 13. This determines the first position. The second measuring probe is moved over the thread axially by hand and is inserted into the first, second, third or other thread course at a distance from the first probe 23. A dial gauge indicates the axial distance, from which the pitch of the thread is determined. With this device, therefore, the axially contacting probe (on the front face of the workpiece) for determining the right measuring position of the thread is lacking. The probe 23 is run rigidly in a thread groove without detecting whether there is a precise radial or axial point contact.
In this reference, no measurement is made as to whether the threaded body is round, whether a cone is present, or of the thread depth and the pitch diameter of the thread. Only superficially precise measurements may be made by the device of Metzger by a simply shaped probe and use of manual shifting.
U.S. Pat. No. 2,938,275 issued to Hofler teaches a device for measuring gears. The device uses two calipers spaced from each other whereby the spacing and the number of teeth between the calipers can be determined. Between these two calipers, over an arc, is a pair of measuring tracers with which the teeth of a gear are measured with respect to their flanks (sides) and also the distance over the arc. The problems in measuring a thread are entirely different from those in measuring a gear. While the teeth of the gear to be measured according to Hofler are always in uniform arrangement around the entire gear on the circumference thereof in a certain gear width, a thread naturally extends spirally over an axial length of a workpiece due to a pitch. This means that the entire measuring process must take place differently in a thread measuring device than in a gear testing device. For this reason, the device taught by Hofler does not solve any of the problems which the present invention attempts to solve.